Abstract
The domestication of animals has generated a set of phenotypic modifications, affecting behaviour, appearance, physiology and reproduction, which are consistent across a range of species. We hypothesized that some of these phenotypes could have evolved because of genetic correlation to tameness, an essential trait for successful domestication. Starting from an outbred population of red junglefowl, ancestor of all domestic chickens, we selected birds for either high or low fear of humans for five generations. Birds from the fifth selected generation (S5) showed a divergent pattern of growth and reproduction, where low fear chickens grew larger and produced larger offspring. To examine underlying genetic mechanisms, we used microarrays to study gene expression in thalamus/hypothalamus, a brain region involved in fear and stress, in both the parental generation and the S5. While parents of the selection lines did not show any differentially expressed genes, there were a total of 33 genes with adjusted p-values below 0.1 in S5. These were mainly related to sperm-function, immunological functions, with only a few known to be relevant to behaviour. Hence, five generations of divergent selection for fear of humans produced changes in hypothalamic gene expression profiles related to pathways associated with male reproduction and to immunology. This may be linked to the effects seen on growth and size of offspring. These results support the hypothesis that domesticated phenotypes may evolve because of correlated effects related to reduced fear of humans.
Highlights
The domestication of animals represents an important event in the history of mankind, and has been used ever since Darwin as a proof-of-principle in evolution [1]
Based on genetic mapping of differences between wild and domestic pigs, Rubin et al [3] suggested that new colour phenotypes are a result of conscious human selection of preferred appearances, whereas Trut et al [4] showed evidence that coloration may change as a secondary response to selection for increased tameness in experimentally domesticated foxes
We explore the possible genetic mechanisms underlying the evolution of the observed suite of behavioural and reproductive traits associated with increased tameness in red junglefowl
Summary
The domestication of animals represents an important event in the history of mankind, and has been used ever since Darwin as a proof-of-principle in evolution [1]. Based on genetic mapping of differences between wild and domestic pigs, Rubin et al [3] suggested that new colour phenotypes are a result of conscious human selection of preferred appearances, whereas Trut et al [4] showed evidence that coloration may change as a secondary response to selection for increased tameness in experimentally domesticated foxes. The latter experiment is one of the most extensive in the field, and the results show that many of the commonly observed domesticated phenotypes, such as increased reproduction, colour and stress sensitivity changed when farm foxes were selected only for reduced fear of humans for a few generations. Among the more recent is the neural crest hypothesis, in which many of the traits associated with the domestic phenotype can be traced back to the role of neural crest cells (NCC) and deficits of NCCs during embryonic development
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